1. Field of the Invention
This invention relates to a tone source circuit for electronic musical instruments which is designed so that a unit phase coefficient corresponding to a note code is read out for each time division channel and accumulated, thereby to simplify the construction of a tone generating part.
2. Description of the Prior Art
A tone source that has been widely employed in electronic musical instruments in the past is such as shown in FIG. 1. A high-frequency clock pulse signal from a main oscillator 1 is frequency divided by a twelve-note frequency divider 2 using frequency ratios respectively corresponding to scales (B to C); the scale pulse outputs are further frequency divided by 1/2 frequency dividers 3.sub.1 to 3.sub.12 so as to obtain respective octave notes; and these outputs are provided to a gate circuit group 4, in which they are subjected to ON-OFF control by a key depression signal from a key switch 5 and from which they are applied via a tone filter group 6 to a sound system 7.
Such a conventional type of tone source circuit is arranged so that tone source signals of the same number as keys are produced simultaneously; therefore, as will be seen from FIG. 1, it requires an enormous amount of wiring and hence is uneconomical in terms of manufacturing cost and steps involved. In this conventional tone source circuit, the number of notes which can be produced at one time is about twelve at most, so that the frequency dividers and the gate circuits except those used for producing the twelve notes remain idle at that time. Further, the increased amount of wiring poses such a problem that the frequency dividers 3.sub.1 to 3.sub.12 and the gate circuit group 4 cannot be fabricated as a large scale integrated circuit on account of a limitation on the number of pins used.
To avoid such shortcomings of the prior art, there has been proposed a system for time sharing a small number of common circuits through utilization of the time sharing techniques; however, this system is difficult to apply to such a twelve-note divider and octave divider system as shown in FIG. 1.